An electromechanical motor with a solid-state actuator drive apparatus is disclosed in EP 1 098 429 BI. A rotatably mounted shaft with a first diameter is encompassed by a drive body with a drive body opening in the form of a cylindrical hole. A shaft surface of the shaft is able to roll on the inner surface of the drive body opening, which has a second diameter which is somewhat larger than the first diameter. At the same time, a shaft axis of the shaft and a central axis of the drive body opening are aligned parallel to one another. The rolling movement and therefore the rotation of the shaft are brought about by a circular sliding movement of the drive body or its axis parallel to the shaft axis. In the simplest case, the circular relative movement between the axis of the drive body opening and the shaft axis is produced by two electromechanical actuators in the form of linear actuators, which in particular can be driven independently of one another and the active directions of which, i.e. directions of the utilized change in length, are orthogonal to one another. In the simplest case, the actuators are disposed so that the active directions span a plane perpendicular to the shaft axis or annular hole axis. By a suitable electrical control of the actuators, one of the actuators is excited to produce a sinusoidal deflection and the other actuator to produce a cosinusoidal deflection with the same frequency and amplitude as a function of time. In doing so, the magnitude of the amplitude of the deflection exceeds one-half of the diameter difference of the first and second diameter, thus reliably overcoming the diameter difference between the drive body opening and the shaft. As long as the linear movements of the two linear actuators are superimposed on one another independently, in total the drive body and shaft are displaced with respect to one another in a circular manner and the rotatably mounted shaft rotates.
If a torque load is applied to the shaft, then a force is transmitted between drive body and shaft, which, in the embodiment described, is by friction. The force transmission can be improved by introducing gearing. As, in this case, no further slip can occur, in addition a very high positioning accuracy and reproducibility of the positioning process is achieved.
The basic design of an embodiment according to the related art is shown in a highly abstract form in FIG. 5. In this case, the force can be transmitted between drive body opening and shaft by friction or by positive locking.
A shaft 1 is rotatably mounted by two radially rigid pivot bearings 2, e.g. sliding bearings, ball bearings or needle bearings, in a bearing holder 3 in each case. The two bearing holders 3 are rigidly connected by two bridge elements 4. Each bridge element 4 provides a mounting for an actuator. The two actuators 5.1, 5.2 are described in the following as X-actuator for a movement in a first direction X perpendicular to a shaft axis Z, and as Y-actuator for a movement in a second direction Y perpendicular to the shaft axis Z and perpendicular to the first direction X. The two actuators 5.1, 5.2 are rigidly mechanically connected to the associated bridge elements 4 on the mounting side. In practical motor design, the functional elements, i.e. the bearing holders and bridge elements, are preferably formed as part of a single-part or multi-part motor housing. A mechanically rigid connection, symbolized by triangles, of one of the bridge elements 4 to a motor holder (not shown), such as a frame or machine bed for example, is therefore correspondingly to be understood as a fixing to a motor housing or carrier element. At their drive ends, the actuators 5.1, 5.2 are mechanically rigidly connected to a drive body 6 with a drive body opening 6.1 in the form of a cylindrical hole. The shaft 1 feeds through or into the drive body opening 6.1.
With such an embodiment, in motor operation, the motor housing with the pivot bearings 2 and the shaft 1 can be considered to be quiescent with regard to translation, and the drive body 6 as moving.
FIG. 5 shows the instantaneous situation in which the Y-actuator 5.2 is just at its maximum deflection and the drive body 6 in the drawing rests against the bottom of the shaft 1. In the left-hand illustration, a plan view, the hidden X-actuator 5.1 is therefore shown distorted. This representation is deliberately greatly exaggerated in order to illustrate the principle. For solid-state actuators used as actuators in practice, an actuator deflection only reaches about 1-2 per mil of the actuator length. With a currently typical actuator length of ca. 30 mm, a maximum of a=60 μm actuator deflection is achieved. Taking into account the necessary assumption that the actuator deflection a must exceed the diameter difference between a second diameter D of the drive body opening 6.1 and a shaft diameter d of the shaft 1, i.e. a>(D−d), it becomes clear that the “bending” of the actuators 5.1, 5.2 perpendicular to their active direction remains negligibly small.
Torques of up to 2 Nm have already been taken off and measured at the shaft 1 in electromechanical motors of this design using a positively locked force transmission between drive body 6 and shaft 1. In this case, the load torque is transmitted by the shaft 1 to the drive body 6, in particular by positive locking, passes from here via the actuators to the motor housing or the bridge elements, and is finally dissipated at the motor holder 7. As the shaft 1 is rotatably mounted in the motor housing and is fitted in the bearing holders, torque cannot as a basic principle be transmitted at the bearing points.
Consequently, the whole load of an active torque M must be absorbed and dissipated by the actuators 5.1, 5.2. As a result, the actuators 5.1, 5.2 are subject to considerable bending. Many known and new actuator materials, and in particular currently used ceramic piezoelectric multi-layer actuators, are mechanically brittle in their behavior. With high torque loads in particular, cracks can therefore initially form in the actuator material with subsequent failure due to breakage.
The bending load is accompanied by the fact that part of the actuator material is subjected to tensile stress and part of the actuator material is subjected to compressive stress. Tensile stresses in particular are highly damaging to brittle actuator materials, such as piezoceramic materials for example. In contrast with this, these materials have a high strength and loading capability with respect to compressive stress.
A first approach to a solution to such a problem relates to an arrangement of linear actuators in pairs, as described in the not yet published DE 10 2005 022 355. Here, a reduction in the mechanical stress in the actuators brought about by torque loads is achieved by increasing the area moment of inertia of the actuator arrangement.
A further approach, which likewise uses the increase in the appropriate area moment of inertia, is described in the not yet published DE 10 2006 032 993 as a design of a production-oriented unit for driving piezoelectric ring motors by piezoelectric multi-layer actuators with rectangular cross section.
Further approaches are concerned with the efficient provision of a high mechanical compressive stress for the actuators, wherein however the compressive stress must not or must only insignificantly hinder their deflection. The compressive stress provided must exceed superimposed components of mechanical tensile stress which occur in operation, so that in total no damaging tensile stress can occur in the actuator material in any conceivable operating state. Such approaches are described in the not yet published DE 10 2006 032 995 in the form of a pre-stressing system for production-oriented mechanical compressive stressing of the piezo actuators in the piezoelectric adjustment drive, and in the not yet published DE 10 2006 032 996 in the form of a circumferential spring wire for production-oriented mechanical compressive stressing of the piezo actuators in the piezoelectric adjustment drive.